The present invention relates to an optical glass and an optical part made of the optical glass. More specifically, it relates to an optical glass that substantially does not contain any Ta2O5 and has high-refractivity low-dispersion optical properties, and various optical parts made of such optical glass.
With the spread of digital cameras, demands for small-sized lenses are more and more increasing in recent years. A high-refractivity low-dispersion glass is suitable as an optical glass material for producing such small-sized lenses, and demands for the high-refractivity low-dispersion glass are therefore increasing as well.
The glass of the above type, for example, an optical glass having a refractive index [nd] of at least 1.8 and an Abbe""s number [xcexdd] of at least 41 contains a large amount of lanthanoids and high-valance components such as Nb2O5 and Ta2O5 that will produce a high refractive index and low dispersion, as is described in JP-B-50-6326.
Tantalum is used as a raw material for Ta2O5 that plays an important role in the above optical glass. Not only the existing amount of the tantalum is small, but also the tantalum is used as a material for a tantalum electrolytic capacitor for use as a part for a cellular phone, and demands for tantalum are therefore increasing with the spread of cellular phones. The price of the tantalum is hence increasing. Under the circumstances, it is therefore difficult to stably provide a large amount of a high-refractivity low-dispersion glass containing a large amount of Ta2O5.
Besides the aspect of the above raw material supply, Ta2O5 intensely works to increase the glass transition temperature [Tg], so that it is required to increase the annealing temperature in a production process. As a result, there is caused problem that the lifetime of an annealing furnace is decreased or that it is made difficult to carry out the continuous operation of the furnace for a long period of time. Moreover, the energy consumption for the annealing is inevitably increased. Further, since a glass of the above type contains a smaller amount of oxides forming a glass network such as SiO2 and B2O3, the viscosity at a liquidus temperature thereof is low, and it is difficult to attain stable production of the glass. For example, when a molten glass is continuously cast into a casting mold from a flow pipe, the pipe is required to have a very small diameter since the glass has a low viscosity, and as a result, the output of the glass is far lower than it is expected in an industrial level.
Under the circumstances, it is an object of the present invention to provide an optical glass that substantially does not contain any Ta2O5 and has high-refractivity low-dispersion properties and various optical parts made of the optical glass stably and at a low cost.
For achieving the above object, the present inventor has made diligent studies and as a result has found that the above object can be achieved by an optical glass having a specific glass composition and having a refractive index [nd] and an Abbe""s number [xcexdd] which satisfy specific relationships, and the present invention has been completed on the basis of the above finding.
That is, according to the present invention, there are provided;
(1) an optical glass having a glass composition comprising, by weight %, 0 to 7% of SiO2, 18 to 30% of B2O3, provided that the total content of SiO2 and B2O3 is 23 to 35%, 2 to 10% of ZnO, 30 to 50% of La2O3, 0 to 20% of Gd2O3, 0 to 20% of Y2O3, provided that the total content of La2O3, Gd2O3 and Y2O3 is 45 to 60%, 3 to 8% of ZrO2 and 3 to 12% of Nb2O5, the total content of these components being at least 95% by weight, the optical glass having a refractive index [nd] and an Abbe""s number which satisfy all of the following relational expressions,
1.795xe2x89xa6ndxe2x89xa61.850
41.0xe2x89xa6xcexddxe2x89xa644.5
ndxe2x89xa7xe2x88x920.01xc3x97xcexdd+2.24,
(2) an optical glass recited in the above (1), wherein the glass composition contains, by weight %, 0.5 to 6% of SiO2, 20 to 30% of B2O3, 2 to 8% of ZnO, 30 to 50% of La2O3, 1 to 15% of Gd2O3, 0 to 10% of Y2O3, 4 to 7% of ZrO2, 4 to 10% of Nb2O5 and 0 to 1% of Sb2O3, and the total content of these components is at least 98% by weight,
(3) an optical glass recited in the above (2), wherein the weight ratio of the total content of La2O3, Gd2O3 and Y2O3: the content of ZnO is from 9.5:1 to 25:1, and the weight ratio of the content of ZnO: the content of Nb2O5 is from 1:1.30 to 1:4.2,
(4) an optical glass recited in the above (1), (2) or (3), which has a liquidus temperature of 1,200xc2x0 C. or lower and has a viscosity of 3 dPaxc2x7s or greater at the liquidus temperature,
(5) an optical glass recited in any one of the above (1) to (4), which has a glass transition temperature [Tg] of 670xc2x0 C. or lower, and
(6) An optical part made of the optical glass recited in any one of the above (1) to (5).